In level measurement, microwaves are transmitted by means of the antenna toward a surface of a filling material, and the echo waves reflected at the surface are received. An echo function which represents the echo amplitudes as a function of the distance is formed, and from this the probable useful echo and its transit time are determined. The distance between the filling material surface and the antenna is determined from the transit time.
It is possible to use all the known methods which permit relatively short distances to be measured by means of reflected microwaves. The best known examples are pulsed radar and frequency modulated continuous wave radar (FMCW radar).
In the case of pulsed radar, short microwave transmitted pulses, referred to below as wave packets, are transmitted periodically, are reflected from the filling material surface and, after a transit time which depends on distance, are received again. The received signal amplitude as a function of time constitutes the echo function. Each value in this echo function corresponds to the amplitude of an echo reflected at a specific distance from the antenna.
In the FMCW method, a continuous microwave is transmitted and is periodically linearly frequency modulated, for example in accordance with a sawtooth function. The frequency of the received echo signal therefore has a frequency difference with respect to the instantaneous frequency of the transmitted signal at the instant of reception, and this frequency difference depends on the transit time of the echo signal. The frequency difference between the transmitted signal and received signal, which can be obtained by mixing the two signals and evaluating the Fourier spectrum of the mixed signal, thus corresponds to the distance of the reflecting surface from the antenna. In addition, the amplitudes of the spectral lines of the frequency spectrum which is obtained by Fourier transformation correspond to the echo amplitudes. This Fourier spectrum therefore constitutes the echo function in this case.
Level measuring instruments which operate with microwaves are used in very many branches of industry, for example in the chemical industry or in the foodstuffs industry. Typically, the level in a container is to be measured. These containers usually have an opening on which a connecting piece or a flange is provided for fastening measuring instruments.
In industrial measurement, dielectric rod antennas and horn antennas are regularly used for transmitting and/or receiving. Typically, use is made of a housing having a housing section which has the geometry of a short-circuited waveguide. Inserted into this housing is an exciter element, via which microwaves can be transmitted and/or received through the housing section. In the case of a horn antenna, a funnel-like section which broadens in the direction of the container and forms the horn adjoins the housing. In the case of the rod antenna, a rod made of a dielectric and pointing into the container is provided.
The interior of the housing is usually virtually completely filled by an insert made of a dielectric. In the case of the horn antenna, the insert has a conical end pointing into the container. In the case of rod antennas, the rod-like antenna adjoins the insert.
In coaxial lines, electromagnetic waves are propagated without dispersion in the transverse electromagnetic mode (TEM mode). This field mode is therefore particularly well suited to transporting wave packets or electromagnetic waves which have a frequency spectrum with a finite but often very great bandwidth. The advantage of dispersion-free propagation is particularly important when the waves or wave packets to be transmitted have the above mentioned frequency bandwidth. Wave packets which are fed in then experience virtually no spreading and, in the case of linearly frequency modulated microwaves, any deviation from linearity is largely avoided.
However, those modes which exhibit a radiation characteristic with a pronounced forward lobe are better suited to the directed transmission of electromagnetic waves by means of an antenna. This property is exhibited, for example, by the fundamental mode, the transverse electric 11 mode (TE-11), which is capable of propagation in circular waveguides. The required mode conversion, for example from the TEM mode into the TE-11 mode, takes place as a result of the injection into the short-circuited waveguide by means of the exciter element.
DE-U 94 12 243 describes a level measuring instrument which operates with microwaves, having
a housing section, PA1 which is designed as a waveguide short-circuited at one side and one end by a rear wall, PA1 which is virtually completely filled with an insert made of a dielectric, PA1 an exciter element, PA1 which projects into the housing section and PA1 which is connected to a microwave source, and PA1 an antenna, adjoining the housing section, for transmitting and/or receiving microwaves. PA1 a housing section, PA1 which is designed as a waveguide short-circuited at one side and one end by a rear wall, PA1 which is virtually completely filled with an insert made of a dielectric, PA1 an exciter element, PA1 which projects into the housing section and PA1 which is connected to a microwave source, and PA1 an antenna, adjoining the housing section, for transmitting and/or receiving microwaves. PA1 a housing section, PA1 an exciter element, PA1 an antenna, adjoining the housing section, for transmitting and/or receiving microwaves, and PA1 a gap arranged in the insert between the exciter element and the antenna,
Here, the exciter element is a transmitting pin which is inserted laterally into the waveguide. In such an asymmetric arrangement, higher modes are excited in a circular waveguide in addition to the desired fundamental TE-11 mode.
EP-A 821 431 likewise describes a level measuring instrument which operates with microwaves, having
The exciter element described is a transmitter wire whose two ends are arranged on the rear wall of the section of the housing and which has three straight segments, one of which runs essentially parallel to the rear wall.
With regard to the desired modes, this form of injection represents a considerable improvement by comparison with the above described lateral injection, but here, too, a proportion, albeit a very small proportion, of higher modes is still generated. This becomes noticeable in the case of pulsed radar, in particular, if very short pulses are generated in that case. The shorter a pulse, the greater the bandwidth of the frequencies contained in it.
The formation of higher modes inevitably leads to the increased occurrence of dispersion effects in the waveguide. At a given frequency, dispersion is very much more pronounced in higher modes than in the fundamental mode.
Higher modes regularly have an unsuitable radiation characteristic and interfere with the directional characteristic of the antenna.
A further disadvantage is that the higher TM-01 mode, by comparison with the fundamental mode, exhibits a long ringing period or decay period. This leads, for example in the case of pulsed radar, to a transmitted pulse not having decayed until after a relatively long period. An echo which occurs in this period can then be detected only when its amplitude considerably exceeds the amplitude of the decaying transmitted pulse. This period predefines a minimum physical distance which must exist between the measuring instrument and the level to be measured. If the distance falls below this physical minimum, reliable measurement is no longer ensured. The minimum distance corresponds to half the path traced by electromagnetic waves during said period, and is usually referred to as the blocking distance.